NOTE: The following post focuses on a 1966 production specific details. This question came up again the other day so thought I would share it with all here in an effort to help others if they run across this.
To add to the stamping date discussion there were times during the production of sheet metal panels and brackets were the worker inserting the individual ingots into the die incorrectly. There are little ones like placing the letter "D" or number "2" upside down so that it is backwards when viewing the rest of the stamps that we see from time to time. They appear to rarely have caught these little mistakes or if they did but were more focused on getting parts stamped and out than these little details.
For some of us that follow, collect and compare these details, these mistakes create sub-patterns within a production period or year, on the individual fender and on the car they were installed on, as we can identify these loads of mis-stamped fenders, after they arrived at the individual car plants.
Such a period took place during 1966 production were a group of fenders were produced with a stamp pattern that did not fit the regular pattern and can lead some to assume that something is incorrect about a fender they may have on their car. The purpose of this post is to inform owners about this finding through the data collected and shared below.
First the driver side fenders in this post are marked 2 91 4 as shown below. This may be the case where the worker reversed the "9" and the "1" or the "2" and the "4" since we know there was not ?91? days in any month and no 4th shift. Pattern shows up at San Jose plant on Mustangs and Shelby?s where other shipments of fenders are dated in March.
From the sample I've collected (about a dozen cars all from scheduled April and May car production period at San Jose there is definitely IMHO a solid identifiable pattern and belief that these are original fenders installed at the plant so if you have a car with this stamping on the driver?s side fenders all of these examples and data should provide you with confidence that it is original to the car from that period.
As of yet I have not found any driver's side front fenders , as described in this post no Mustangs built at Dearborn or Metuchen but would love to hear about any findings of such. Its possible that all of the fenders that day from that die were sent to only one plant.
Just four examples from four cars from the period identified above. Same pattern and same somewhat deep stamping on all of them